The functional principle of the Coriolis mass flowmeter is that the measuring tube having medium flowing through it is excited to an oscillation, wherein the direction of oscillation of the measuring tube and thus also of the medium flowing through the measuring tube has at least one component orthogonal to the direction of flow of the medium in the measuring tube. The orthogonal component of oscillation of the medium flowing in the measuring tube causes a Coriolis inertia in the flowing medium, which works against the orthogonal component of the oscillation. The Coriolis inertia causes the occurrence of a phase difference between the oscillation of the measuring tube at two respective measuring tube points along the longitudinal axis of the measuring tube, which is proportional to the mass flow of the medium through the measuring tube. The Coriolis mass flowmeter measures the phase difference and, from this, determines the mass flow of the medium.
Each oscillation sensor is arranged at a measuring tube point, has a first sensor connection and a second sensor connection and is designed for output of a sensor signal representing the oscillation at the measuring tube point between the first sensor connection and the second sensor connection. The measuring tube points are spaced from one another along the longitudinal axis of the measuring tube. More than one of the oscillation sensors can also be arranged at each measuring tube point, wherein the oscillation sensors arranged at one of the measuring tube points are considered a single oscillation sensor. The sensor signal is an analog, electrical signal, which is output between the first sensor connection and the second sensor connection and wherein the amplitude of the sensor signal represents the amplitude of the oscillation and the phase of the sensor signal represents the phase of the oscillation at the measuring tube point.
The evaluation unit has a digitization unit having at least a first digitization channel and a second digitization channel. Thereby, each of the digitization channels has at least a first analog signal input. The digitization unit digitizes the analog signals applied to the digitization channels at certain points in time. The points in time are usually determined by the evaluation unit. The digitization unit normally has at least an analog-to-digital converter for digitization. An analog-to-digital converter converts the amplitude of an analog, electrical signal applied to one of its signal inputs at a certain point in time into a piece of data corresponding to the amplitude.
Each of the sensor signal paths has an output signal path and an input signal path. The beginning of each of the output signal paths is located in the evaluating unit and the end of each of the output signal paths is connected to the respective first sensor connection by one of the oscillation sensors. The beginning of each of the input signal paths is connected to the respective second sensor connection of one of the oscillation sensors and the end of each of the input signal paths is connected to the respective first analog signal input of one of the digitization channels.
The beginning of each of the sensor signal paths coincides with the beginning of the respective output signal path and the end of each of the sensor signal paths coincides with the end of the respective input signal path. Accordingly, each of the sensor signal paths also includes the signal path between the first sensor connection and the second sensor connection of the respective oscillation sensor.
The sensor signal of the first oscillation sensor propagates in the direction of the first analog signal input of the first digitization channel of the digitization unit from the second sensor connection of the first oscillation sensor over the first input signal path. Accordingly, the sensor signal of the second oscillation sensor propagates in the direction of the first analog signal input of the second digitization channel from the second sensor connection of the second oscillation sensor over the second input signal path.
The evaluation unit is designed for determining a mass flow of a medium flowing through a measuring tube from a phase difference caused by the flow of the medium between at least a sensor signal of the first oscillation sensor and a sensor signal of the second oscillation sensor. The digitization unit digitizes the analog sensor signals so that they are available to the evaluation unit for further processing as digital sensor signals. The evaluation unit usually determines the phase difference from the digital sensor signals. The phase difference between two signals of the same frequency corresponds to a time difference of the occurrence of an arbitrarily determined same amplitude of the two signals. In particular, the amplitude of the signal at which the slope is steepest is suitable for determining the time difference. In the case of bipolar, symmetrical, harmonic signals, this is the zero crossing.
In Coriolis mass flowmeters of a similar type known from the prior art, the problem arises in practice that the sensor signal propagation times differ in the sensor signal paths. The temporal difference between the sensor signal propagation times in each of two of the sensor signal paths is called sensor signal propagation time difference. A sensor signal propagation time difference is not to be differentiated from the time difference corresponding to a phase difference, which is why sensor signal propagation time differences compromise the accuracy of Coriolis mass flowmeters of a similar type. Sensor signal propagation time differences occur, in particular, in sensor signal paths of different lengths. Signal paths of different lengths, for example, result when the evaluation unit determines the phase difference and the evaluation unit and the oscillation sensors are separate units and the spacing between the oscillation sensors and the evaluation unit differ from one another.